Patent Document 1 describes a spark-ignition direct-injection engine injecting fuel into a combustion chamber configured as a cavity which is recessed from a top surface of a piston. This direct-injection engine accelerates combustion by adding ozone to intake air which is introduced into a cylinder. As a fuel spray, which is injected through a fuel injection valve arranged at a bore center of the cylinder, distributes in the combustion chamber, flames, which spread from a central portion toward a peripheral portion of the combustion chamber, combust before reaching a wall surface of the cavity. As a result, a gas layer which does not contribute to combustion forms between a combustion gas in the central portion of the combustion chamber (i.e., inside the cavity) and the wall surface of the cavity. This may significantly reduce cooling loss. The direct-injection engine described in Patent Document 1 is further configured to have a high geometric compression ratio of higher than or equal to 15, and significantly improves thermal efficiency in combination with the above-described reduction in cooling loss.
Albeit not disclosing a technique for reducing cooling loss such as that described in Patent Document 1, Patent Document 2 describes how a ceiling portion of a cylinder head is provided with a fuel injection valve at an exhaust side, and with an active species generating device, which generates radicals by non-thermal plasma discharge and which is adjacent to the fuel injection valve at an intake side. The active species generating device generates radicals in an ignition chamber deep inside the combustion chamber. In this engine, gas flow, which accompanies fuel injection from the fuel injection valve adjacent to the active species generating device, is employed to generate negative pressure in the vicinity of an opening of the ignition chamber and to draw the radicals generated in the ignition chamber into the combustion chamber.
Although not disclosing a technique for reducing cooling loss in the same way as that described in Patent Document 2, Patent Document 3 describes how a ceiling portion of a cylinder head is provided with a fuel injection valve at an exhaust side, and with a spark plug adjacent to the fuel injection valve at an intake side. In this engine, a main injection is followed by a second injection in the scope of which the fuel injection valve injects fuel around a discharge gap of the spark plug. By this, a secondary voltage required for the spark plug to produce an electric spark is lowered.